1. Field of the Invention
The invention relates generally to the field of gas chromatography and specifically to the design of chromatographic systems that increase the maximum operating temperature and lifetime of gas chromatograph columns.
2. Statement of the Problem
Gas chromatography is a physical method for the separation, identification, and quantification of chemical compounds. A sample mixture is injected into a flowing neutral carrier stream and the combination then flows through a tube or chromatographic column. The inner surface of the column is coated or packed with a stationary phase. As the sample mixture and carrier stream flow through the column, the components within the mixture are retained by the stationary phase to a varying degree depending on the relative volatility of the individual components and on their respective affinities for the stationary phase. When the individual mixture components are released into the carrier stream by the stationary phase, they are swept towards the column outlet where they are detected and measured with a detector. Different chemical compounds are retained for different times by the stationary phase. The specific compounds in the mixture can be identified and their relative concentrations determined by measuring peak retention times and peak areas respectively.
Gas chromatograph (GC) measurements are facilitated by the application of heat to the chromatographic column to change its temperature. The use of heated column ovens in GC systems greatly increases the number of compounds that can be analyzed and reduces the time required for each analysis by increasing the volatility of higher molecular weight compounds.
Many methods have been described for heating chromatographic columns. The most commonly used methods utilize air bath ovens that circulate heated air through a thermally insulated chamber in which a chromatographic column is placed. Once an analysis has been completed, cold ambient air is circulated through these ovens to cool them. Air bath ovens typically utilize large fans and/or moving baffles to control the flow of warm or cold air through the oven enclosure and thereby control the temperature of the column. U.S. Pat. No. 4,050,911 to Welsh, U.S. Pat. No. 4,070,169 to Iwao et. al., U.S. Pat. No. 4,096,908 to Lamy, U.S. Pat. No. 4,181,613 to Welsh et. al., U.S. Pat. No. 4,599,169 to Ray, U.S. Pat. No. 4,752,216 to Hurl, U.S. Pat. No. 4,771,628 to Sisti et. al., U.S. Pat. No. 5,744,029 to Li et. al., U.S. Pat. No. 5,830,262 to Marchini et. al., and U.S. Pat. No. 6,126,728 to Walsh et. al. describe different types of GC air bath ovens.
Polyimide coated, fused silica capillary columns are the most commonly used columns in gas chromatography. They are strong, flexible, and lightweight. They can often be used for thousands of analyses. However, in existing gas chromatographs these columns are not suitable for analyses above 350xc2x0 C. because the polyimide coating rapidly oxidizes above 350xc2x0 C. making the columns brittle. Hydrocarbon separations of molecules containing in excess of 50 carbon atoms are generally performed at temperatures exceeding 350xc2x0 C. For these high temperature analyses, chemists must either frequently replace polyimide columns that break after tens of analyses or use significantly more expensive metal clad columns. Even at temperatures below 350xc2x0 C., the lifetime of polyimide-coated fused silica columns is reduced by polyimide oxidation in an oxygen environment.
In the absence of oxygen, the thermal stability of polyimide is substantially enhanced thereby significantly increasing its maximum useful operating temperature. The lifetime of polyimide can be increased by over an order of magnitude at high temperature in an oxygen-free environment. This can in turn dramatically increase the maximum operating temperature and lifetime of polyimide-coated GC columns. However, it is not practical with commercially available GC column oven technology to heat columns in an oxygen-free (inert gas) environment. Conventional ovens are designed to utilize vast volumes of circulating atmospheric air to heat and cool columns.
Complete process GC""s are often built within purged, explosion proof cabinets. A positive barometric pressure is typically maintained within these cabinets to prevent the ingress of potentially explosive gas from an external industrial environment. These purged cabinets are pneumatically connected to a source of pressurized, clean (non-explosive) air that maintains the pressure within the cabinet at a level above the prevailing atmospheric pressure. Because these cabinets are generally large and because inert gas is expensive as compared to clean air, inert gas is not generally used to purge explosion proof process GC cabinets. Moreover, oven/column cool down times are generally poor in enclosed, purged GCs because vast volumes of cold air are not readily available to cool down the column ovens. As most process GC""s keep the column oven at a fixed temperature, cooling time limitations are unimportant. However, for the broad range of GC""s and analyses that utilize temperature cycling, slow cool down is a problem. It results in fewer analyses per day, thus increasing costs. Maintaining a complete GC within a purged, inert gas filled cabinet is not a practical method for increasing the high temperature durability of polyimide coated columns. This solution is more expensive than is the problem.
Some GC column ovens have been described which are pneumatically sealed from the atmosphere outside the oven enclosure. See for example, U.S. Pat. No. 4,286,456 to Sisti et. al. and U.S. Pat. Nos. 6,093,921 and 6,157,015 to Gaisford et. al. None of these devices, however, can maintain an inert gas environment around the column within the column oven enclosure.
3. Solution
Pneumatically sealing a column oven enclosure isolates the interior of the oven enclosure from the atmosphere outside the enclosure. Filling the sealed oven enclosure with inert gas results in an oxygen free environment within the oven enclosure. Polyimide coated, GC columns heated in such an oven enclosure will last much longer at elevated temperatures and can be used at higher temperatures than can the same columns in conventional GC column ovens. Furthermore, suitably designed column oven systems achieve cooling times at least as fast as those of conventional GC ovens using only modest volumes of inert gas.
The present invention provides a gas chromatography system that maintains an inert gas within an oven enclosure during the heating process. The system comprises a pneumatically sealed oven enclosure, an inert gas supply, and an exhaust gas control system. The oven enclosure includes a sealed access port through which columns can be installed, two sealed sample line ports through which sample lines enter and exit the oven enclosure, and two gas ports through which gas may enter or exit the oven enclosure.
It is the object of the invention to provide a gas chromatograph system that increases the maximum operating temperature of a gas chromatographic column.
It is the object of the invention to provide a gas chromatograph system that increases the lifetime of a gas chromatographic column.
It is the object of the invention to provide a gas chromatograph system that pneumatically isolates the interior of an oven enclosure from its exterior.
It is the object of the invention to provide a gas chromatograph system that heats a gas chromatographic column in an inert gas environment.
It is the object of the invention to provide a gas chromatograph system that maintains an oxygen free environment within a column oven by maintaining a positive pressure of inert gas within the oven enclosure.
It is the object of the invention to provide a gas chromatograph system that uses cool, flowing inert gas to accelerate the cooling rate of a column.
These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.